Ultrasound techniques and apparatus are now well established as tools for a variety of medical diagnoses. Ultrasound waves are transmitted from an ultrasound transmitter in a selected direction into a patient's body and, in one type of diagnosis, are scattered therein by changes in the mass density of the tissues encountered, the typical result being a variation in the velocity at which the ultrasound wave is transmitted through different tissues such as bone or cavity interfaces. Some of the ultrasound waves are reflected back to a receiving device used in conjunction with the ultrasound transmitter, and the return signals are processed in any of several known ways to produce useful images of the body structures under study. When an ultrasound wave being transmitted through the body encounters moving elements, e.g., red blood cells in motion, then the returned signal that reaches the signal receiver will have experienced a change in frequency by an amount which corresponds to the magnitude of the velocity component of the moving element along the direction of the incident sound beam. Such a shifting of the frequency of an incident wave due to motion of either the source or of the moving body reflecting the incident wave gives rise to the commonly known Doppler effect.
Clinical ultrasound imaging systems routinely include Doppler processing systems, i.e., data processing and comparisons systems, which enable a user to make estimates of blood velocity in various vessels of a patient's body. See, for example, Hatle, L, and Angelsen, B, "Doppler Ultrasound in Cardiology", Lea and Febiger, Philadelphia, 1982, for a discussion of such well-known clinical Doppler technology.
Blood flow velocity distributions within the chambers of the heart, through and around the heart valves and in the major vessels near the heart, provide highly useful data to cardiologists as they attempt to assess heart and heart valve performance. The hemodynamic performance of a heart valve is reflected in the pressure gradient across the valve when it is open and in the extent of reverse flow, i.e., regurgitation, which occurs when the valve closes. In vivo pressure gradient measurements of this type historically have been made by invasive techniques, e.g., by cardiac catheterization.
High pressure gradients are known to be collated with the functioning of a stenotic valve. This is because the resistance offered by the valve to flow therethrough increases as the opening available for the blood flow becomes smaller due to disease or body deterioration. See, for example, McDonald D. A., "Relation of Pulsatile Pressure to Flow in Arteries", J.Physiol. 127, 533, 1955. Since blood is an incompressible fluid at body pressures, for a particular volume flow rate of the blood the required fluid velocity must increase as the level of stenosis increases. Measurement of blood flow velocity therefore enables a cardiologist to estimate the pressure gradient across the stenotic valve and, hence, the level of stenosis under study.
Doppler ultrasound techniques and apparatus offer a cardiologist a convenient and non-invasive means for estimating the level of stenosis if the requisite blood flow velocity determinations can be made accurately. Furthermore, the volume of blood pumped by the heart in each beat, i.e., the stroke volume, as well as the regurgitant volume, may be estimated by combining analysis of the Doppler data and imaging information. See, for example, Hatle, et al., supra. It is believed that approximately one million such cardiac ultrasound examinations are performed each year in the United States.
Replacement of damaged or otherwise unsound cardiac valves often provides the best solution for the patient, and it is believed that as many as 90,000 such operations are performed world-wide every year. Although such cardiac valve replacements are successful in saving many lives, significant complications are frequently encountered in the implantation of prosthetic valves. Among the most serious complications of this type are thrombo-embolism, hemolysis, post-operative infections, and mechanical failure of the prosthetic valve itself. The most common problem is thrombo-embolism, which is encountered at the rate of about five events per one hundred patient years, the problem being dependent upon the dynamics of blood flow through the valve that is involved. See, for example, Woo, Jr. and Yoganathan, A. P. "In Vitro Pulsatile Flow Velocity and Turbulent Shear Stress Measurements in the Immediate Vicinity of Prosthetic Heart Valves", Life Support Systems 4, pp 47-62, 1986.
It is now generally accepted that understanding of how blood flow patterns cause cell damage, thrombus growth and embolism has become an important aspect of research into improved prosthetic valve design. In addition, further research is also needed in determining the mechanical properties and characteristics of prosthetic valves and such valves are tested by phonocardiographic procedures in which prosthetic or natural valve sounds are captured by sensitive microphones and analyzed to detect valve malfunctions.
It is thus now clear that for ultrasound examinations of both natural as well as prosthetic heart valves, and for both hydrodynamic and acoustic evaluations of prosthetic valves before and after their implantation into a patients body, there is a need for test equipment which can simulate human use and reaction, permit detailed hemodynamic measurements, enable sensitive detection of valve sounds, and allow clinical-type ultrasound examinations. The desired test equipment should include a left heart simulator which produces pulsatile fluid flow in a fluid selected to mimic human blood under a low pressure head of 8-10 mm. Hg. through the mitral valve to fill a left ventricular cavity, the fluid being then pumped through an aortic valve within the pressure range simulating that of the human arterial system, i.e., typically 120 mm. Hg. peak systolic to 80 mm. Hg. end diastolic.
It is very important that the simulated pulsatile flow used in the test equipment be consistent in successive beats, i.e., that the pressure distributions from beat to beat be reproduced consistently. It is also highly desirable to have the stroke volume, the pressure on both sides of the valve, the stroke rate, and the systolic/diastolic time ratio adjustable over a range of physiological valid conditions and that each of these parameters be accurately measurable by standard measurement means. Likewise, in any such test equipment it is highly desirable that there be provided "windows" which enable a user to perform ultrasonic imaging of the valves from positions which approximate anatomical locations and views of clinical interest. It is also highly desirable to have available the facility of test means to determine in detail the fluid velocity distributions downstream of the valves under study for comparison with the Doppler measurements being made through the various ultrasound windows.
The known prior art includes at least some pulse duplicator systems which respond to some aspects of these perceived needs. Such prior art includes a general purpose system produced by The Dynatek Corporation, as described by, for example, Swanson, W. M. and Clark, R. E., "A Simple Cardiovascular System Simulator: Design and Performance", J. Bioeng. 1: 133-145, 1977, which is intended to allow testing of candidate prosthetic valves but includes no capability for performing velocity measurements or ultrasound examination of the valves. Also described by Philpot, E. F., in "In vitro flow Visualization and Pressure Measurement Studies in the Pulmonary Artery", Thesis, Georgia Institute of Technology, Atlanta, June 1985, is a system for visualization of cardiac flows.
U.S. Pat. No. 4,894,013, to Smith et al titled "Anthropomorphic Cardiac Ultrasound Phantom", issued on Jan. 16, 1990, teaches apparatus that includes multiple ultrasound windows for examination of left ventricular flows but includes no mechanism for performing corresponding and contemporaneous fluid velocity field measurements. There are also known a number of Doppler flow phantoms which are intended to permit calibration of ultrasound Doppler systems for flows through straight tubes. These include, for example, Reid, J. M., "Methods of Measuring Performance of Continuous Wave Doppler Diagnostic Equipment", Draft IEC Standard, Subcommittee 29D, Working Group 10, 1983; McDicken, E. N., "A Versatile Test Object for the Calibration of Ultrasonic Doppler Flow Instruments", Ultrasound Medical Biology, 12:245, 1986; Reid, J. M., "Report on Performance Testing of Ultrasonic Doppler Diagnostic Equipment", Draft IEC Report, Technical Committee 87, Ultrasonics Working Group 10, Doppler Devices, 1988 and Boote, E. J. and Zabzebski, J. A., "Performance Tests of Doppler Ultrasound Equipment With a Tissue and Blood Mimicking Phantom", J. Ultrasound Med. 7:137-147, 1988. None of these Doppler flow phantom systems, however, are able to produce flows of a blood mimicking fluid that have characteristics equivalent to those of cardiac blood flow in human beings.